Immunoreactive gamma-melanotropin in rat pituitary and plasma: a partial characterization.

A RIA for the gamma MSH region of proopiomelanocortin has been established and validated. The antiserum was raised to synthetic bovine gamma MSH, and it cross-reacts well with larger polypeptides, including gamma MSH and 16K fragment, which contain the gamma MSH sequence. Gel filtration chromatography of extracts prepared from the anterior and neurointermediate lobes of rat pituitary and from rat plasma reveal two heterogeneous immunoreactive (IR-) forms of gamma MSH in each venue which elute in molecular weight peaks of approximately 11,000 (11K) and 6,000 (6K). Addition experiments indicate that the smaller material is not an artifact generated from the larger form during preparation. By the criterion of retention on affinity columns of Concanavalin A-agarose, these peptides are glycosylated. The 6K form represents 8-17% of the total IR-gamma MSH in the anterior lobe and about 30% of that in the neurointermediate lobe. IR-gamma MSH are released by dispersed rat pituitary cells in response to several of the same secretagogues which modulate the secretion of other proopiomelanocortin-derived peptides. Changes in the plasma and anterior lobe content of IR-gamma MSHs and ACTH resulting from perturbation of the pituitary-adrenal axis are concordant. In particular, the plasma concentration of 6K IR-gamma MSH increases dramatically after imposed stress.